Verification of FGGE Assimilations of the Tropical Wind Field: The Effect of Model and Data Bias

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Abstract We examine the tropical wind field analyses produced by a recent assimilation of the Final FGGE II-b dataset. Our aim is to estimate the effects, on the tropical wind analyses, of biases in the data and biases in the assimilation system. The assimilation system was similar to that used operationally at ECMWF in the first half of 1985. The period studied is the first Special Observing Period (SOP-1). Important differences occur in the intensity of divergence at upper and lower levels in the western Pacific, as measured by cloud-track winds (SATOBs) and by rawinsondes (TEMPs). There appear to be important biases also in the SATOB estimates of the zonal flow at upper and lower levels in the eastern Pacific. There are substantial biases in the wind directions at some west African stations. The 6-hour forecasts which provide the background fields for the analyses show important underestimates of the mean intensity of the tropical divergence field, particularly in the equatorial western Pacific. The er...